The present invention relates generally to ceramic foams which are highly advantageous for the filtration of molten metals such as molten aluminum, and particularly to efficient and economical methods for their preparation.
Porous ceramic foams obtained by treating impregnated open-celled organic sponges or foams have been disclosed in the prior art, as represented by U.S. Pat. No. 3,090,094 to Schwartzwalder et al., U.S. Pat. No. 3,097,930 to Holland, and U.S. Pat. No. 3,111,396 to Ball. Furthermore, the use of such ceramic foams as filters for molten metal, and particularly for the filtration of molten aluminum and copper, has been disclosed in U.S. Pat. No. 3,893,917 issued July 8,1975 for "Molten Metal Filter" and in U.S. Pat. 3,947,363 issued Mar. 10, 1976, by Michael J. Pryor et al.
As noted above, the prior art suggests the preparation of ceramic foam materials. Thus, U.S. Pat. No. 3,111,396 to Ball suggests that an organic polymer foam after immersion into a suspension of refractory material may subsequently be compressed by passage through rolls to effect the removal of excess refractory. This technique, which is comparable to a wide variety of conventional expulsion techniques used in the art suffers from an inherent disadvantage in that the slurry is not uniformly distributed through the body of the article. Thus, the outer area of the article tends to be more thinly coated with slurry than that near the center line. Such defects are particularly evident at the extremes of the permeability range found suitable for use in the preparation of filters for molten metal; thus, bodies of high permeability tend to have undesirably weak surfaces and edges, while bodies of relatively low permeability tend to exhibit undesirable center line blockage. Such defects render the resulting foams unsuitable for use in the filtration of molten metal.
The above difficulties were overcome by the process disclosed in our prior application Ser. No. 589,294, filed June 23, 1975, referred to above and the disclosure of which is incorporated herein by reference. This required an initial step of impregnation of the polymer foam with the slurry of ceramic composition, wherein the foam was subjected to repeated compression and recovery while completely immersed in the vigorously stirred or vibrated slurry. The impregnated foam was then removed from the impregnating tank and treated to remove excess slurry by passing it through two sets of rolls to effect twice-repeated compression and recovery, the roll gaps being preset to accomplish first a compression of 50-90% and then a compression of 70-90%. However, such process was disadvantageous in requiring an excessive amount of handling of the polymeric foam slabs, which could deleteriously affect the distribution of the contained slurry and also the strength properties of the completed ceramic foam body. Also, the procedural steps could not readily be automated or be adapted for use in a continuous or semi-continuous mass production line.
Accordingly, it is a principal object of the present invention to provide an efficient and economical method for the preparation of ceramic foam articles having predetermined permeability properties, wherein the operations of impregnation of polymeric foam with a slurry of ceramic composition and the removal therefrom of excess slurry are combined.
It is a further object of the present invention to provide an efficient and economical method as aforesaid which yields products possessing improved strength properties.
It is yet a further object of the present invention to provide a readily automated method for the production of ceramic foam articles as aforesaid which exhibit structural uniformity and freedom from defects such as center line blockage and outer surface weakness.
It is a still further object of the present invention to provide a method as aforesaid, which lends itself to rapid commercial-scale production techniques through the provision of a unitary operation wherein the steps of slurry impregnation and of excess slurry removal are effected in rapid succession.
Other objects and advantages will be apparent from a careful review of the ensuring description.